A public land mobile network (English full name: Public Land Mobile Network, PLMN for short) is a network providing mobile communications services to the public by a government-authorized operator. When a terminal device is currently in a network different from a home public land mobile network (English full name: Home PLMN, HPLMN for short), the terminal device is in a roaming state and roams in a visited public land mobile network (English full name: Visited PLMN, VPLMN for short).
According to stipulations of 3GPP TS23.122, in general cases, a terminal device needs to run on an HPLMN or on an equivalent home public land mobile network (English full name: Equivalent HPLMN, EHPLMN for short) defined in a subscriber identity module (English full name: Subscriber Identity Module, SIM for short) card. After leaving a coverage area of the HPLMN or EHPLMN, the terminal device may choose to camp on a VPLMN.
To provide uninterrupted mobile communications services to users, different operators deploy base stations in areas including border areas on a large scale. Coverage areas of base stations on both sides of a border usually overlap. When located in the border area, the terminal device may receive signals sent from multiple PLMN base stations. When a coverage rate of an HPLMN operator of the terminal device on a regional border is lower than that of a VPLMN operator of the terminal device on the regional border, the terminal device located in an area belonging to the HPLMN may often camp on the VPLMN.
According to the stipulations of 3GPP TS 23.122, to prevent the terminal device from staying on the VPLMN for a long time, the terminal device probably returns to an HPLMN network from a VPLMN network only in two scenarios: The terminal device is disconnected from the VPLMN, or a higher priority PLMN search timer (High priority PLMN search timer) expires. The higher priority PLMN search timer exists in a higher priority PLMN search period EF-HPPLMN field of the SIM card. That is, when the terminal device is disconnected from the VPLMN, an ordinary network search is performed; or when duration of the higher priority PLMN search timer exceeds a preset value, a higher priority PLMN search is performed. In actual application, however, for example, in an HPLMN border area with a densely deployed VPLMN, it is not easy for the terminal device camping on the VPLMN to disconnect from the VPLMN. The higher priority PLMN search timer is stipulated in the 3rd Generation Partnership Project (English full name: 3rd Generation Partnership Project, 3GPP for short) TS 23.122. The duration of the higher priority PLMN search timer is set in the higher priority PLMN search period EF-HPPLMN field of the SIM card. The duration of the higher priority PLMN search timer ranges from 6 minutes to 8 hours in 6-minute steps, and specific duration of the timer is related to a value of the EF-HPPLMN field. For example, when the value of the EF-HPPLMN field is 0, it indicates that the higher priority PLMN search timer is not set; when the value of the EF-HPPLMN field is 1, timing duration of the higher priority PLMN search timer is 6 minutes; when the value of the field is 2, it indicates that the timing duration of the higher priority PLMN search timer is 12 minutes; and so on. If the EF-HPPLMN field of the SIM card has no value, the timing duration of the higher priority PLMN search timer is set to 60 minutes by default. That is, minimum timing duration of the higher priority PLMN search timer is 6 minutes.
As a result, if the higher priority PLMN search is performed only when the higher priority PLMN search timer expires, the terminal device is very likely to camp on the VPLMN network for a long time, that is, the terminal device goes back from the VPLMN network at a relatively slow speed to camp on the HPLMN network.